1. Field of the Invention
This invention relates to a lithium ion secondary battery in which positive and negative electrodes face each other sandwiching a separator which keeps an electrolytic solution, more particularly to a battery structure in which electric connection between a positive electrode and a negative electrode (electrodes) and separator is improved so that it can be made into thin form and the like optional forms and to a production method for the formation of said structure.
2. Description of the Related Art
There is a growing demand for the miniaturization and lightening of portable electronic instruments, and it is essential to improve performance of batteries to meet such a demand. Because of this, development and improvement of various batteries have been attempted in recent years with the aim of improving the battery performance. Expected characteristics of batteries to be improved include high voltage, large energy density, tolerance for large load resistance, optional shaping, safety and the like. Particularly, lithium ion battery is a secondary battery which can realize the highest voltage, largest energy density and tolerance for largest load resistance among existing batteries, and its improvement is still being made actively.
As its main composing elements, the lithium ion secondary battery has a positive electrode, a negative electrode and an ion conducting layer inserted between these electrodes. In the lithium ion secondary batteries which have been put into practical use, a plate-shaped material prepared by mixing powder of a lithium-cobalt oxide or the like active material with powder of an electron conducting substance and a binder resin and coating the mixture on an aluminum collector is used as the positive electrode, and another plate-shaped material prepared by mixing powder of a carbonaceous active material with a binder resin and coating the mixture on a copper collector is used as the negative electrode. Also, a porous film such as of polyethylene, polypropylene or the like filled with a lithium ion-containing non-aqueous solution is used as the ion conducting layer.
For example, FIG. 7 is a sectional view showing the structure of a prior art cylindrical lithium ion secondary battery disclosed in JP-A-8-83608 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In FIG. 7, 1 is an armor case made of stainless steel or the like which also serves as a negative electrode terminal, 2 is an electrode body contained in the armor case 1, and the electrode body 2 has a structure in which a positive electrode 3, a separator 4 and a negative electrode 5 are coiled in a spiral shape. In order to maintain electric connection among the positive electrode 3, separator 4 and negative electrode 5, it is necessary to apply external pressure to surfaces of the electrode body 2. Because of this, contact among all surfaces is maintained by putting the electrode body 2 inserted into a strong metal case. In the case of a square battery, strips of electrode body are tied up into a bundle and put into a square metal case, thereby pressing them with external force.
As described in the foregoing, in the currently available lithium ion secondary batteries, strong armor cases made of metals and the like are used as a means to closely adhere positive and negative electrodes. Without the armor case, the electrodes are peeled off, so that it becomes difficult to maintain electric connection between the electrodes via an ion conducting layer (separator) and the battery characteristics therefore are deteriorated. On the other hand, not only the energy density of the battery itself is reduced because of the large weight and volume of the armor case occupying entire portion of the battery, but also shapes of the battery are limited due to rigidity of the armor case itself, thus causing a difficulty in making optional shapes.
In view of such backgrounds, development of a lithium ion secondary battery which does not require a strong armor case has been attempted with the aim of achieving lightening and thinning the battery. The point of the development of such a armor case-free battery is how to maintain electric connection of a positive electrode, a negative electrode and an ion conducting layer (separator) which is sandwiched by them, without applying external force. As such a connecting means which does not require external force, a method has been proposed in which the electrodes are closely adhered to the separator making use of a resin or the like.
For example, JP-A-5-159802 discloses a production method in which an ion conductive solid electrolyte layer and positive and negative electrodes are integrated into one body by their heat treatment using a thermoplastic resin binder. In this case, the electrodes are closely adhered to each other by integrating the electrodes and the electrolyte layer into one body, so that electric connection between the electrodes is maintained and the integrated body functions as a battery without applying external force.
Since the prior art lithium ion secondary batteries are constructed in the aforementioned manner, a battery which uses a strong armor case to ensure adhesiveness between electrodes and a separator and electric connection between electrodes is disadvantageous in producing a battery having large energy density, because the ratio of volume and weight of the non-electricity generating part armor case to the entire battery portion becomes large. Also, though a method in which electrodes are closely adhered to an ion conducting body via an adhesive resin has been proposed, it causes a problem in that ionic conduction resistance inside the battery cell increases and the battery characteristics are reduced due to large resistance of the adhesive resin layer, when a solid electrolyte layer is closely adhered to the electrodes simply via the adhesive resin.
In addition, in the case of the battery of JP-A-5-159802, the electrodes are bonded to a solid electrolytic layer with a binder, but since interfaces of the electrodes and the electrolytic layer are covered with the binder, it is disadvantageous in terms of ionic conductivity when compared for example with a case in which liquid electrolytes are used. Even if a binder having ionic conductivity is used, a material having an ionic conductivity equal to or larger than that of liquid electrolytes is not generally known, so that it causes a problem in that battery performance similar to that of a battery in which liquid electrolytes are used cannot be obtained easily.